Ballast structures are well known for use with fluorescent lamps to provide power therefor by controlling the lamp current. Typically, these ballasts have been magnetic ballasts and are relatively large and relatively heavy, for example, between one and two and one half pounds, depending on whether they are step dimmable or continuously dimmable. Weight and size normally are not major considerations for home, industrial or commercial uses. Efficiency of the latter ballast structures likewise is not a major concern, nor is long life and replacement of ballast structures. Magnetic ballasts have an efficiency in the range of about 60%.
It is also known in the prior art to use a unit known as a Cuk converter in power supplies and which operates on a DC power input and provides a controlled DC output. That and similar type units do not convert the controlled DC power to 400 Hz.
In the case of aircraft, however, somewhat different criteria apply. In such uses, long life, weight, size, efficiency, easy replacement and the general requirements for aircraft use present conditions which are far more demanding than the conditions that apply in home, industrial or commercial use. For example, for each pound in reduction in aircraft weight, there is a cost savings of between $400.00 to $500.00, not to mention the reduction in operating costs. Moreover, lamp life is important, more so than in other uses. Further, the need to protect the ballast from faults such as short circuits, shorted or open filaments, open circuits or rectifying lamp loads present unique considerations with respect to ballasts for use in aircraft, especially those intended to operate at relatively high efficiency.
In some cases, the ballast structures are designed for the life of the aircraft, typically 60,000 hours flight time, and operate from a 115 V AC 400 Hz power source. Thus, the reliability demanded for aircraft ballast systems is far greater than for conventional home, industrial or commercial ballasts. The number of ballast systems may vary with aircraft type and configuration. For example, in the case of a Boeing 737, the number of ballasts may vary between 20 and 21 units, depending upon configuration of the aircraft. In the case of a Boeing 757, the number of ballast systems is nominally about 28 units. Thus, the weight due to conventional magnetic ballasts alone may be between 20 pounds to 52.5 pounds for the 737 and between 28 to 70 pounds in the case of the 757 type aircraft. A ballast weight of about 0.71 pounds represents a weight savings of 5.8 and 38.3 pounds in the case of a 737 type aircraft and between 13.8 and 55.8 pounds for the 757 type aircraft. The cost savings is between $2,300.00 and $22,320.00 per aircraft solely on the basis of weight reduction of the ballast. Further weight savings are recognized if the efficiency of the ballast is increased thus allowing a reduction in the size of the power generating equipment. An efficiency of between 85% and 91% as compared to 60% is a significant efficiency increase.
Another problem which arises in aircraft and other uses is that the input power sometimes contains frequencies other than the fundamental input frequency. For example, in the case of aircraft and other generating sources where the desired input frequency is desired to be 400 Hz, it is not uncommon to find odd and even harmonics such as 800 Hz, 1200 Hz, 1600 Hz, 2000 Hz, up to about 3600 Hz and the like, with variations such as 820 Hz, 1220 Hz and so forth. When such frequencies are present at the 400 Hz input, even in amounts as small as 3% to 4%, the result is that beat frequencies occur and, depending upon the frequency, these beat frequencies may fall within a band of frequencies to which the human eye is especially sensitive, causing what is known as "flicker" in fluorescent lamps, a particularly objectionable condition because it is quite visually perceptible. In other cases, these beat frequencies cause variations from the desired frequency in those instances in which precise control of the output frequency is required. Moreover, in the case of aircraft, the radiated EMI from the power control system is a factor and the same should be reduced.
It is also known that the lamp current needed for maximum normal brightness varies with lamp type and may be in the range of 100 milliamps (mA) to 300 mA or more, depending on lamp type. It is also a fact that in the course of normal use, the lamps wear, sometimes in an uneven manner in that one of the hot cathodes may begin to decay before the other, either due to use or some anomaly in the manufacture of that particular lamp. When this takes place, some ballast systems tend to increase the current in order to maintain brightness with the result that power flowing through the ballast is increased, sometimes approaching or exceeding the design characteristics of the ballast. When this takes place, ballast life is markedly reduced. In other cases, the current to the lamp is somewhat greater than needed, overdriving so-called, again with a reduction in lamp life, loss of efficiency and reduction in ballast life and reliability.
It is also known in the prior art that there is a broad range of applications in which a need for control of the magnitude of AC power is required. These applications include such items as incandescent lamp dimmers, motor controllers, AC power supplies and any application requiring the control of AC power, especially in combination with the need for high efficiency, light weight and high reliability, only to mention a few.
In accordance with this invention, many of the prior art shortcomings previously discussed are overcome.